1. Field of the Invention
The present invention relates to processes for infiltrating metal matrices with a binder. In particular, the present invention relates to processes for infiltrating tungsten carbide matrices with metal or metal alloy binders. More specifically, the present invention relates to processes for manufacturing earth boring drill bits which include infiltrating a tungsten carbide matrix with a high-strength metal or metal alloy binder.
2. Background of Related Art
Conventionally, earth boring drill bits that include a powdered or particulate refractory material matrix, which are also referred to as particulate-based drill bits, have been manufactured by processes such as sintering and infiltration. Conventional infiltration techniques typically include the formation of a somewhat porous matrix of powdered or particulate material and the infiltration thereof by powder metallurgy processes. U.S. Pat. No. 3,757,878, which issued to Wilder et al. on Sep. 11, 1973, and U.S. Pat. No. 4,780,274, which issued to Barr on Oct. 25, 1988, each disclose exemplary powder metallurgy techniques that have been conventionally employed to produce infiltrated drill bits. In these powder metallurgy processes, a wear-resistant matrix powder, such as a tungsten carbide powder, is placed into a carbon or graphite mold, forming a particulate-based bit body matrix therein. A steel bit blank is typically inserted into the mold and partially within the particulate-based matrix.
The mold is then placed into a furnace, and a funnel having an infiltrating alloy therein, which is typically referred to as a binder or infiltrant, is placed in the furnace and positioned over the mold. As the furnace is heated, the binder melts and is introduced into the mold. The molten binder penetrates the porous matrix by capillary action and gravity, filling the spaces, or pores, between the matrix powder particles. Conventional furnaces, which heat the mold, funnel and the contents thereof without radiated heat, are typically employed in such infiltration processes. The use of conventional furnaces is, however, somewhat undesirable from the standpoint that a relatively long period of time is typically required when radiated heat is employed to heat the funnel, mold and binder to a temperature that is sufficient to effect infiltration of the porous matrix.
Conventionally, copper-based alloys have been employed to bind matrices which include tungsten carbide particles. Copper-based binders typically have a melting temperature of about 1,065.degree. C. to about 1,200.degree. C. U.S. Pat. No. 5,000,273, which issued to Horton et al. on Mar. 19, 1991, discloses a copper-based alloy binder having a melting temperature of less than 1,050.degree. C. The use of binders with as low melting temperatures as possible is typically desired in order to facilitate the attachment of thermally stable polycrystalline diamond compacts (PDCs) to the bit body during infiltration.
The infiltrated matrix is then cooled. Upon cooling, the infiltrating alloy binder solidifies, binding the matrix powder particles together to form a bit body, and binding the bit body to the steel blank to form a drill bit. Typically, cooling begins at the periphery of the infiltrated matrix and continues inwardly, with the center of the bit body cooling at the slowest rate. Thus, even after the surfaces of the infiltrated bit body matrix have cooled, a pool of molten binder may remain in the center of the bit body, which may generate stress gradients, such as shrinkage porosity or cracks, through the infiltrated matrix, which will likely weaken or damage the bit body.
After the bit body has cooled, a threaded connection may be machined on, attached to, or otherwise associated with the steel blank of the drill bit in order to permit attachment of the drill bit to a drill string. Similarly, cutters and gage trimmers, both of which are typically made of natural diamond or synthetic diamond (e.g., PDCs), nozzles, or any other components that are associated with a finished drill bit may be attached to the bit body or otherwise associated therewith. Such components are typically attached to or associated with infiltrated bit bodies by brazing or welding. Thermally stable PDCs, which are typically referred to as thermally stable products (TSPs), may be assembled with the particulate-based bit body prior to infiltration, and attached thereto or associated therewith during infiltration by the infiltrating binder.
As noted previously, copper alloys may be employed as a binder in infiltrated tungsten carbide bit bodies. Conventional copper infiltrated tungsten carbide bit bodies have high wear-resistance and high erosion-resistance relative to steel bit bodies, and copper-based alloys may be melted at temperatures which permit attachment of cutting elements comprising TSPs to the particulate-based bit body during infiltration thereof. Copper and the copper alloys that are typically employed to infiltrate drill bits are relatively low strength, low-toughness materials. Typically, a copper alloy that may be used to infiltrate a tungsten carbide bit body matrix will withstand up to approximately 30 in-lbs of impact without fracturing, as measured by the Charpy impact strength test. The toughness of the copper alloy infiltrated tungsten carbide bit bodies is even lower when measured by the Charpy impact strength test (e.g., about 2 ft-lbs, Charpy unnotched) or the transverse rupture strength test, and when compared with the strength of the copper alloy itself. Copper alloy-infiltrated tungsten carbide bit bodies can also crack upon being subjected to the impact forces that are typically encountered during drilling. Additionally, thermal stresses from the heat generated during fabrication of the bit or during drilling may cause cracks to form in the bit body. Typically, such cracks occur where the cutting elements have been secured to the matrix body. If the cutting elements are sheared from the drill bit body, the expensive diamonds on the cutter element are lost, and the bit may cease to drill.
U.S. Pat. No. 5,662,183 (the "'183 patent"), which issued to Zhigang Fang on Sep. 2, 1997, discloses an alternative, high-strength binder material and method of infiltrating a matrix of particulate refractory material, such as tungsten carbide, with the binder. The high-strength binder of the '183 patent includes a cobalt-, nickel-, or iron-based alloy. As is known to those in the art, however, cobalt and iron, when molten, may dissolve, or "attack," carbon. Thus, during infiltration of the porous matrix with binders which include significant amounts of these metals, the graphite funnels and molds that are typically employed in the infiltration of tungsten carbide bit bodies may be attacked and destroyed by cobalt, iron, or metal alloys which include these metals during exposure thereto. Increased amounts of damage to the funnel and mold may occur with prolonged exposure to binders which include cobalt or iron. Such damage to the graphite mold may result in an undesirably shaped bit body. Consequently, the product yield of bit bodies that include cobalt- or iron-based alloy binders and that are infiltrated in graphite molds may be low, or the bit bodies may require further processing, such as machining, to remove excess material from the bit face. Lower product yields and additional processing both increase manufacturing costs.
The infiltration method of the '183 patent includes coating the graphite funnel and mold with hexagonal-structure boron nitride (HBN) in order to prevent metals such as nickel, cobalt, or iron from attacking the graphite. Nevertheless, HBN does not completely prevent metals such as cobalt and iron from attacking the graphite. Thus, the use of HBN may not significantly increase product yields or significantly decrease manufacturing costs. Moreover, in order to adequately protect the graphite mold, the layer thickness of the coating material would likely be relatively thick, which would likely alter the dimensions and cutter placement of a drill bit to be defined thereby.
A method of infiltrating a refractory material matrix with a binder which includes cobalt or iron and which reduces or eliminates any damage that may be caused to the mold by the molten binder is needed, as well as an infiltration process which occurs over a shorter period of time and increases product yield.